Understanding Permanent Magnet MotorsFeb 20, 2017
Controlling the speed of ac motors is accomplished using a variable frequency drive (VFD) in most cases. While many scenarios involve using VFDs with induction motors with stator windings to generate a rotating magnetic field, they also can achieve precise speed control using speed or position feedback sensors as a reference to the VFD.
In some situations, it is possible to obtain comparably precise speed control without the need for feedback sensors. This is made possible using a permanent magnet (PM) motor and a process called the "high-frequency signal injection method."